#include <cmath>
#include <iostream>
#include <string>
using namespace std;
using ll = long long;

int main() {
    int n;
    string s;
    cin >> n >> s;
    int ans = 0, zero = 0, one = 100;
    for (const auto c : s) {
        if (c == '0') {
            if (one == 1) {
                ans++;
                zero--;
            }
            zero++;
            one = 0;
        } else {
            const auto p = zero / 3;
            const auto q = zero % 3;
            ans += 2 * p;
            if (q == 2) {
                ans++;
            }
            if (q != 1) {
                one++;
            }
            zero = 0;
            one++;
        }
    }
    cout << ans << endl;
    return 0;
}